1. Field of the Invention
The present invention relates to an ATM cell multiplexer, and in particular to an ATM cell multiplexer connected between terminal equipments and an ATM switchboard. "ATM (Asynchronous Transfer Mode)" technology is expected as a communication technology for realizing a multi-media society, and has been developed for fast data used in an inter-computer communication etc.
However, as seen from a recent ATM forum, a network is demanded in the market where the communication protocols of the prior art such as for voices, HDLC (High level Data Link) and FR (frame relay) can also be interworked with the ATM technology.
When such a network is constructed, it is useful to set up an ATM cell multiplexer as an apparatus in branch circuit or line system in order to construct an ATM switchboard (ATM switch) as a key network and to increase the accommodation efficiency of circuits with terminal equipments.
2. Description of the Related Art
FIG. 1 shows a general arrangement of such an ATM cell multiplexer. An ATM cell multiplexer generally designated by reference numeral 2 is adapted to assemble data received from a terminal equipment generally designated by reference numeral 1 in form of an ATM cell, to send the data to an ATM switchboard generally designated by reference numeral 3 through a trunk circuit, and to disassemble the ATM cell (hereinafter simply referred to as the cell) which is received through the trunk circuit from the ATM switchboard 3 and transmitted to the terminal equipment 1.
In this ATM cell multiplexer, although strict traffic control (QoS control) proper to the ATM protocol is demanded, the prior art ATM switchboard and ATM cell multiplexer installed an ATM switch (not shown) to perform the traffic control.
However, the ATM switch is disadvantageous in that the manufacturing cost is highly expensive and the control of the software/hardware is complicated, requiring a large-sized unit.
Moreover, in a system where cells are simply multiplexed, when the cells through plural connections mutually compete in the ATM cell multiplexer, some of the cells must be delayed or stood by to make the cells communicate without colliding in each of service categories such as CBR (Constant Bit Rate), VBR (Variable Bit Rate), and UBR (Unspecific Bit Rate).
This is not particularly allowed in a service category like CBR in which a time delay is strictly prohibited.